Although conventionally various systems are proposed as an FM demodulator, a PLL system is also one of such systems. The FM demodulator of the PLL system comprises: a voltage controlled oscillator VCO whose oscillating frequency is determined by a variable capacitance (varactor) diode; a phase comparator for receiving an FM signal and a frequency signal from the VCO and for comparing their phases; a low-pass filter for integrating an output of the phase comparator; and a closed-loop circuit for applying an output voltage of the low-pass filter as a control voltage to the varactor diode of the VCO. Once in lock, the VCO frequency tracks the frequency changes of the FM signal. Therefore, the control voltage to be applied to the varactor diode of the VCO corresponds to the frequency deviation of the FM signal, i.e., to the FM modulating signal, so that an FM demodulated signal is obtained. Such a PLL FM demodulator has advantages such that: a wide demodulation band is obtained; the characteristic is stable independently of a tuning circuit; it has a spurious suppression ability by which necessary signals are mainly demodulated, or the like; however, there is a problem on its linearity. In other words, this is because since the change ratio of the control voltage to the VARI-CAP diode vs. the change ratio of the capacitance is not ideal, this would be distortion components of the demodulated signal.
It is therefore an object of the present invention to dissolve the above-mentioned drawbacks and to provide a PLL demodulator with little distortion.